Proton exchange membrane fuel cells (PEMFC) are considered to be one of the most promising sources of clean electrical energy for the near future. PEMFC are electrochemical generators which produce direct electrical current from a gaseous fuel (typically hydrogen, pure or in admixture) and a gaseous oxidant, normally consisting of oxygen or air. The core component of the cell is the membrane-electrode assembly, consisting of an ion-exchange membrane, which is the solid electrolyte supporting the whole process and the physical separator of the anode and cathode cell compartments, bonded or otherwise coupled to gas diffusion electrodes. The gas diffusion electrodes, one cathode and one anode contacting either side of the ion-exchange membrane, usually comprise a gas diffusion medium and a catalyzed layer.
Several kinds of technical solutions are known from the prior art for these components: the catalyzed layers are in some cases applied to the gas diffusion media prior to the coupling with the membrane, and/or they are directly coated on the membrane surface prior to the application of uncatalyzed gas diffusion media thereon. The gas diffusion medium usually comprises an electrically conductive web and one or more gas diffusion layers. The conductive web can be metallic or carbon-based, and may consist of a metal mesh, foam or cloth, of a woven or non-woven carbon cloth, of a carbon paper or of any other preferably porous or perforated medium.
Gas diffusion layers are provided to supply suitable paths for the diffusion of gaseous reactants inside the electrode structure toward the catalytic sites whereupon the electrochemical reactions of fuel oxidation (anode side) and oxidant reduction (cathode side) occur. They are usually based on mixtures of electrically conductive inert fillers (for instance, carbon particles) and suitable, preferably hydrophobic binders (for instance, PTFE or other fluorinated binders). Gas diffusion layers should be carefully designed to provide a permeable and smooth structure, to ensure a correct apportionment of the gaseous reactants without incurring heavy mass transport penalties, and to provide a good contact with the membrane.
Improved gas diffusion structures for fuel cells are for instance disclosed in U.S. Pat. No. 6,103,077. A catalyzed layer can then be applied to the gas diffusion layers, for instance as described in U.S. Pat. No. 6,017,650. Catalyzed layers of the prior art comprise noble metal catalysts such as platinum, optionally supported on carbon or graphite particles, a suitable binder, which can be the same hydrophobic binder already present in the gas diffusion layers, and an ionomeric component, usually an ionomeric perfluorocarbon species. The ionomeric component can be added to the catalyst-binder mixture and/or it can be applied subsequently as an external layer wetting the pre-applied catalyst and binder particles. Gas diffusion electrodes of this kind, coupled to proton-exchange membranes known in the art, for instance based on fluorocarbon acids such as Nafion® (a trademark of U.S. company DuPont), give rise to membrane-electrode assemblies characterized by excellent performances. Nevertheless, the noble metal component is exploited to such a low extent in structures of this kind, that very high specific loadings are required (usually in the range of 0.3 to 1 mg/cm2 of platinum both for the anode and for the cathode side in commercially available products). The high amount of noble metal required for obtaining suitable performances in fuel cells is perhaps the single most important factor preventing PEMFC (and other types of fuel cells such as DMFC, direct methanol fuel cells) from having a commercial success.
Direct metallization of ion-exchange membranes with a catalyst layer has been proposed as a means to achieve a better catalyst-membrane interface, allowing a better catalyst exploitation and therefore, the use of lower noble metal loadings. However, no means for direct metallization of membranes has proven effective and practical up to now. High temperatures required by sputtering or ultra high vacuum deposition (UHV) are destined to impart consistent damages to the delicate ion-exchange membranes, and even the common physical and chemical vapor deposition techniques (PVD or CVD) have proven too difficult to control and cumbersome to scale up.
A substantial improvement in the metallization of membranes is disclosed in U.S. Pat. No. 6,077,621, wherein the use of dual IBAD is proposed for this purpose. Dual IBAD, which is an evolution of the Ion Beam Assisted Deposition (IBAD) technique, has the advantage of being a low temperature process and very easy to scale up. The membrane is initially cleaned and textured by a first low-energy ion beam, for instance, an Ar+ beam, having an energy not higher than 500 eV; a second beam is then focused on the membrane, containing higher energy ions (such as O2+ or N2+) together with the ions of the metals to be deposited, previously evaporated by means of an electron beam. Dual IBAD is much advantageous also over conventional IBAD (in which a single beam is used), in that it allows the formation of a better controlled film with the required density and porosity while imparting a minimum stress to the membrane structure.
Since the handling of a large sized ion-exchange membrane in a continuous metallization process is not very easy, a further improvement of this technique has been disclosed in U.S. Pat. No. 6,673,127. In this case, a very thin ion-exchange membrane layer is formed on a gas diffusion structure, and then subjected to dual IBAD. Although this technique allows to obtain high power densities in fuel cells with reduced platinum loadings, it still presents some disadvantages that the present invention wishes to address. Firstly, although the performances of these electrodes can be high, they can be somewhat unpredictable since the reliability of this technique is affected by the characteristics of the ionomer film, which can vary according to the preparation conditions. The state-of-the-art liquid ionomer film is of fluorocarbonic nature, since this is the only known ionomeric material that would allow high power density operation, and it has to be recast from an alcoholic or hydroalcoholic suspension of a fluorocarbon acid such as the product commerciallised as “Liquid Nafion” by Dupont.
The nature of these suspensions is not always consistent, since average molecular weight, morphological parameters of the suspended particles, rheological parameters and other factors may vary in a remarkable fashion from one batch to the other. Moreover, also in the best cases, the utilization factor of the catalyst with liquid ionomer-embedded particles does never approach unity. Liquid ionomers for gas diffusion electrodes were first described in U.S. Pat. No. 4,876,115 as a means for extending the proton conduction paths within the interstitial spaces of a three-dimensional catalytic layer, thereby improving the utilization factor of the catalyst (which is a measure of the availability and accessibility of the catalyst itself as a site for the desired reaction.) This approach is effective up to a certain extent, only mimicking the ideal situation whereby all the catalyst is present in a very thin and smooth, quasi-two-dimensional layer, in direct contact with the membrane surface.
Besides solving the issue of lowering the platinum loading (or more generally the noble metal loading) in fuel cell electrodes, another problem which should be addressed is the low stability of fluorocarbon-based ionomeric components in membrane-electrode assemblies at certain process conditions. In some applications (such as automotive ones), fuel cells are operated in a discontinuous fashion depending on the instant power demand. Since PEMFC are known for their very quick start-up and their remarkable ability of following the requirements of steeply variable power demand, thay are the most promising candidate for operating in this field. However, in conditions of zero or near-zero power demand, i.e. when little or no current is generated (open circuit voltage conditions), a consistent generation of peroxides on the anode side is likely to take place. Perfluorocarbon materials are often unstable in these conditions, especially over long times. Also for this reason, alternative membranes (for instance based on polybenzimidazole, polyetherketones or polysulphones) have been developed for fuel cell applications.
In any case, none of these materials has proven suitable for being employed as a proton conducting material for the electrode interface according to the teaching of U.S. Pat. No. 4,876,115 and perfluorocarbon materials such as the aforementioned “Liquid Nafion” are always used. The elimination of this component would therefore be beneficial for many reasons, not only of cost and reliability, but also of overall chemical stability at certain process conditions.
For all the above reasons, direct metallization of gas diffusion media was attempted with several different techniques in the past. U.S. Pat. No. 6,159,533 claims that excellent performances are obtainable with a PVD deposition of platinum on a gas diffusion medium, even though the examples show that the actual recorded performances don't go beyond a modest 732 mA/cm2 at 0.358 V in a fuel cell provided with a very thin membrane (20 microns), fed with a very high gas flow-rate (3.5 stoichiometric ratio on air, 2 stoichiometric ratio on pure hydrogen) at a relatively high pressure (about 2 bar).
A more interesting result was obtained with the invention disclosed in the co-pending U.S. Provisional Patent Application Ser. No. 60/580,739, consisting of a gas diffusion medium, free of ionomeric components, provided with a noble metal coating by means of a dual IBAD deposition. The electrochemical performances detected in a fuel cell with this type of electrode and a Nafion 112 ion-exchange membrane (0.3 A/cm2 at about 0.8 Vand 0.7 A/cm2 at about 0.7 V feeding pure hydrogen and air at 1.5 bar a, at a stoichiometric ratio of 2 and with a cell temperature of 80° C.) are certainly closer to those expected for a real industrial application. However, some undesired limitations can be noticed with this type of electrode at higher current densities (around 1 A/cm2), as cell voltage tends to drop suddenly due to the onset of diffusive limitations. Most likely, the diffusion rate of the gaseous reactants through the noble metal coating obtained by dual IBAD is not sufficient to sustain operation above a certain current density.